The wheels and axles on automotive vehicles are suspended by springs that support the mass or weight of the vehicle. The springs absorb the road shock as the wheels encounter uneven road surface conditions and keep the jarring, up-and-down action of the axles from being transmitted to the vehicle frame and body.
Only the parts of the vehicle attached to the frame derive benefit from the spring action and are commonly referred to as "sprung" weight. "Unsprung" weight is the term used to describe those parts of the vehicle not protected by the spring action and subject to the full force of gravity, save protection offerred by the tires for the reduction of road shock. Front and rear axles are examples of "unsprung" weight parts. As "unsprung" weight increases, so does the roughness of the vehicle ride. For example, if unsprung weight equals sprung weight there is, in effect, no spring action. It is, therefore, desireable to maintain unsprung weight as a low percentage of overall vehicle weight.
The ride of a vehicle, then, depends for the most part on the spring action. Springs that are too soft allow too much movement of the axle while springs that are too stiff do not allow enough movement of the axle. in either case the result is poor vehicle ride quality.
The softness or stiffness of a spring is determined by its deflection rate, i.e., the force required to produce 25 mm of spring deflection. The ideal automotive spring is designed to have a substantially constant deflection rate over all operating conditions while remaining within the elastic limits of the spring material.
Ideals, however, are seldom possible and as a compromise to produce a satisfactory ride for a given vehicle type, relatively soft springs are utilized and complimented with shock absorbers which act as dampeners to control spring osscillation.
While the above described approach to automotive suspension system design is standard within the industry it is not devoid of shortcomings. For example, the combination of spring and shock absorber typically provides a satifactory ride quality but not the optimum ride quality for any given vehicle. Nor can the standard design accommodate changing vehicle loading conditions which, in the case of pick-up trucks and vans may vary significantly at differing times during normal use with the consequent variation in vehicle ride height and suspension geometry.
Over the years much inventive ingenuity has been applied to the resolution of these shortcomings. One such proposed solution is to be found in the air bag suspension. This system is comprised of an air filled, heavy rubber bag encased in a metal dome or girdle. The air bag replaces the conventional spring, is filled with compressed air--typically from an engine driven compressor--and supports the vehicle mass. As the vehicle wheels encounter bumps or holes in the road surface, th air in the bag further expands or compresses.
This sytem alleviates some of the shortcomings of the conventional suspension in that it is possible to vary, either manually or automatically, through a series of special level control valves, the riding qualities of the vehicle to compensate for heavy or uneven loading or to increase or decrease the vehicle ride height as te driver desires. The riding quality of the vehicle may be adjusted by altering the amount of air pressure in the bag.
Given the benefits to be derived from such air activated suspensions systems compared to the conventional suspension systems it comes as no surprise that the air system continues to attract the interest of inventors and examples of such systems may be readily found in the patent literature, such as Canadian patent numbers 971583 which issued to Lear Siegler inc. in July 1975; 9615 16 which issued to Dura Corp in January 1975, and U.S. Pat. No. 4,310,172 which issued to Claude, G. T. et al, in January 1982.
All such air activated systems heretobefore known are designed either as a direct replacement for conventional springs or are intended as a replacement for the conventional shock absorber and as such are utilized only with coil springs.